Merlantean

Merlantean Decks use the discarding costs of "Mermail" monsters with the destruction and searching effects of "Atlantean" monsters to clear the opponent's field while also maintaining field presence and hand advantage.

"Genex Undine" is a versatile monster that can send any WATER monster from the Deck to the Graveyard as a cost to add "Genex Controller" to your hand, being able to use any of the "Atlantean" effects directly from the Deck. With "Genex Controller" in your hand, running "Allure of Darkness" is also an option. Another card that has viable usage in this build is Water Hazard, which can Special Summon a Level 4 or lower Mermail or Atlantean monster, using the effects of Abysspike or Abyssturge because they activate upon Normal or Special Summon.

"Atlantean" monsters have their effects triggered when they're sent to the Graveyard to activate a WATER monster's effect. "Atlantean Heavy Infantry" and "Atlantean Marksman" destroy a face-up or face-down card your opponent controlls, respectively. "Atlantean Dragoons" searches for any Sea Serpent-Type monster from your Deck, regardless of its Level, serving as the main search engine in your Deck, as most of your monsters are Sea Serpents. Also, all of them have effects while on the field.

A useful Field Spell Card to mix into this deck is Lemuria, the Forgotten City, which not only boosts all WATER monsters ATK and DEF by 200 but can also give your those WATER monsters the versatility to Xyz Summon powerful monsters like Number 9: Dyson Sphere or Super Dimensional Robo Galaxy Destroyer.

"Mermail Abysspike" can discard a WATER monster to add 1 Level 3 WATER monster from your Deck to your hand. While "Atlantean Dragoons" can't search for cards like "Genex Undine" and "Mermail Abysslinde", this card can.

One of the strongest monsters in this Deck, "Mermail Abyssmegalo", can be Special Summoned from your hand by discarding 2 WATER monsters. While triggering up to 2 Atlantean effects, it can also add 1 "Abyss-" Spell/Trap from your Deck to your hand, usually "Abyss-sphere". And while in the field, it can tribute 1 WATER monster to attack twice, allowing for the use of Atlanteans that might be on the field. Using two can also result in a Xyz Summon for Number 11: Big Eye.

Finally, one of the cards that are used to finish your opponent is "Moulinglacia the Elemental Lord", for being searchable by "Atlantean Dragoons".

While not focusing on it, this Deck can also make use of Synchro Summons by using "Deep Sea Diva" and "Genex Controller". "Gungnir, Dragon of the Ice Barrier" is one of the main monsters to be summoned this way, being able to destroy up to 4 cards on the field by using the Atlanteans.